For any airship designed for long-endurance operation, one factor affecting endurance is the leakage of lifting gas. To overcome this problem, the airship can be designed to minimize the leakage rate of lifting gas (e.g., by proper hull design), or it can drop ballast as the lifting gas is depleted, or it can carry a reservoir of lifting gas in compressed form. The reservoir can be used to replenish lifting gas as required, until the reservoir is depleted.
In a tethered airship system such as disclosed in co-pending application Ser. No. 13/159,215, there is a strong motivation to keep the upper airship as light-weight as possible. If a way can be found to replenish the lifting gas with a relatively small on-board reservoir (with its associated tanks or other equipment for providing the lifting gas), or no on-board reservoir at all, the size of the upper airship and the weight of its hull can be minimized.
This invention is directed to the problem of external replenishment of lifting gas in a tethered airship system, although the invention may find other applications.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.